Steel sheet for double wound pipe and method of producing the pipe

ABSTRACT

A steel sheet for double-rolled tubes has excellent formability, and excellent strength and toughness after forming and heat treatment of a tube because of suppressed coarsening of the ferrite grain size and a method for making the same comprises: hot finish-rolling of a steel material containing C: 0.0005-0.020 wt %, and one or two of Nb: 0.003-0.040 wt %, and Ti: 0.005-0.060 wt % at a final temperature of 1,000-850° C., coiling at 750° C. or less, cold rolling, continuous annealing at 650° C.-850° C. for 20 seconds or less, and second cold-rolling at a rolling reduction rate of 20% or less, so that at least one of Nb and Ti is present in a solid solution state in an amount of 0.005 wt % or more, and the crystal grain size in the ferrite structure is in the range of 5 to 10 μm.

This application is a 371 of PCT/JP97/04289 filed Nov. 25, 1997.

TECHNICAL FIELD

The present invention relates to a steel sheet suitable for adouble-rolled tube and a method for making the same, in which thesurface of the steel sheet is plated with copper or a self-brazingmetal, shaped into a pipe, and heated to a temperature higher than themelting point of the plating metal for a short duration to form thedouble-rolled tube.

BACKGROUND ART

Double-rolled tubes, having excellent appearances similar to those ofcopper tubes and excellent thermal characteristics, as well as highstrength and toughness due to steel, have been used in the fields ofconnection tubes for various compressors and brake tubes of vehicles.

Double-rolled tubes are described in detail in, for example,"TETSU-TO-HAGANE", No. 1, p. 130 (1980). A typical method for making adouble-rolled tube will now be described in brief. Using a cold-rolledsteel sheet having a thickness of approximately 0.30 mm, both faces ofthe steel sheet are electroplated with copper. Next, the steel sheet isfurled such that the rolling direction of the steel sheet is parallel tothe central axis of the tube. The steel sheet is furled double so thatthe thickness of the tube is double that of the steel sheet. The tube isheated to a higher temperature than the melting point of copper for"self-brazing" which represents bonding of the steel sheet walls bymeans of filling the gap between the walls with molten copper. Adouble-rolled tube is prepared in such a manner. Next, cold reformingand sizing are performed to obtain a final product.

As described above, double-rolled tubes generally require reliabilitysuch as air-tightness in view of their usage.

Since steel sheets used for double-rolled tubes are ultra-thincold-rolled steel sheet having a thickness of 0.35 mm or less andrequire significantly excellent formability, box-annealed low-carbonsteel sheets generally have been used.

Since the box-annealed sheets are relatively soft materials and haveexcellent formability, these can be satisfactorily used as raw materialsfor double-rolled tubes. The sheet, however, requires several days forproduction, and thus has an inferior production efficiency. Anotherdisadvantage is non-uniformity of the mechanical properties in thelongitudinal and transverse directions of the coil. In addition, inorder to reduce abrasion of the die for forming the tube and in order toimprove shape fixability in the tubing process (furling process), softermaterials having excellent formability, while maintaining high strength,are demanded.

Ultra-low-carbon steel sheets having a significantly decreased carboncontent (0.020% or less) have been noted in the field of generalcold-rolled steel sheets. The ultra-low-carbon steel sheets are suitablefor a continuous annealing process having a high production efficiencyand creating excellent uniformity of the mechanical properties. Further,the steel sheets are soft and have excellent formability. The use of acontinuously-annealed soft ultra-low-carbon steel sheet is in prospectfor solving the above-mentioned problems.

In the production process of the double-rolled tube, however, a coldworking of approximately 7% to 8% strain is applied to the steel sheetafter tubing by drawing. Further, the tube is subjected to heattreatment for self-brazing at a higher temperature than the meltingpoint (1,083° C.) of copper, although for a short duration. Thus,coarsening of the micro-structure in the steel during the forming andannealing is anticipated. When a double-rolled tube is formed of anultra-low-carbon steel sheet, presence of coarse grains severelyaffecting the strength and toughness of the double-rolled tube are oftenobserved.

It is an object of the present invention to solve the above-mentionedproblems involved in conventional technologies, and thus to provide acold-rolled steel sheet suitable for producing double-rolled tubeshaving self-brazing characteristics, as well as significantly improvedmechanical properties compared to conventional materials, and highproduction efficiency and uniformity of mechanical properties, and amethod for making the same.

It is a particular object of the present invention to provide acold-rolled steel sheet suitable for producing double-rolled tubeshaving the following characteristics, and a method for making the same:

1) Deterioration of its characteristics, particularly strength andtoughness due to coarse grains does not occur during the heat treatmentfor self-brazing;

2) The steel sheet has a low deformation resistance in the tubeproduction process to minimize abrasion of a die and thus to prolong itslife;

3) The steel sheet is soft during the production of the tube and hasexcellent shape fixability;

4) The final tube has sufficiently high strength, ductility, andtoughness; and

5) The steel sheet is an ultra thin sheet with a thickness of 0.35 mm,has excellent uniformity of mechanical properties in the longitudinaland transverse directions of the steel sheet (steel strip), and has novariation in the shape.

The present inventors have discovered that containing a given amount ormore of nonprecipitated Nb or Ti is effective for preventing the growthof grains contrary to conventional knowledge that control of theprecipitates is effective, as a result of intensive experimentation andstudy for solving the above-mentioned problems.

Further, by controlling the annealing condition within an adequaterange, as well as limiting steel components, and hot-rolling conditions,such as the final temperature at finish-rolling and the coilingtemperature, the present inventors have discovered that the given amountor more of nonprecipitated Nb or Ti is secured in a nonprecipitatedstate, that is, a solid solution state, that the crystal grain size iscontrollable within an optimum range, and that mechanical properties arestabilized after heat treatment in the tube production process, and thepresent inventors have completed the present invention.

DISCLOSURE OF INVENTION

1) The present invention relates to a steel sheet for double-rolledtubes having excellent formability, and excellent strength and toughnessafter forming and heat treatment of a tube comprising:

C: 0.0005-0.020 wt %; and further comprising one or two of

Nb: 0.003-0.040 wt %, and

Ti: 0.005-0.060 wt %;

at least one of Nb and Ti being present in a solid solution state in anamount of 0.005 wt % or more, the grain size in the ferrite structurebeing in the range of 5 to 10 μm (Claim 1).

2) Further, the present invention relates to a steel sheet fordouble-rolled tubes having excellent formability, and excellent strengthand toughness after forming and heat treatment of a tube comprising:

C: 0.0005-0.020 wt %,

S: 0.02 wt % or less, and

N: 0.0050 wt % or less; and further comprising one or two of

Nb: 0.003-0.040 wt %, and

Ti: 0.005-0.060 wt %;

each of the excessive Nb and Ti contents, calculated based on theassumption that TiN, TiS, TiC and NbC are formed as much as possible inthat order, being less than 0.005 wt %, at least one of Nb and Ti beingpresent in a solid solution state in an amount of 0.005 wt % or more,the crystal grain size in the ferrite structure being in the range of 5to 10 μm (Claim 2).

3) Also, the present invention relates to a steel sheet fordouble-rolled tubes, described above in 1) or 2), comprising:

C: 0.0005-0.020 wt %,

Si: 0.10 wt % or less,

Mn: 0.1-1.5 wt %,

P: 0.02 wt % or less,

S: 0.02 wt % or less,

Al: 0.100 wt % or less, and

N: 0.0050 wt % or less; and further comprising one or two of

Nb: 0.003-0.040 wt %, and

Ti: 0.005-0.060 wt %; and

the balance being Fe and incidental impurities (Claim 3).

4) Further, the present invention relates to a steel sheet fordouble-rolled tubes, described above in 1) or 2), comprising:

C: 0.0005-0.020 wt %,

Si: 0.10 wt % or less,

Mn: 0.1-1.5 wt %,

P: 0.02 wt % or less,

S: 0.02 wt % or less,

Al: 0.100 wt % or less, and

N: 0.0050 wt % or less; and further comprising one or two of

Nb: 0.003-0.040 wt %, and

Ti: 0.005-0.060 wt %; and

at least one selected from the group consisting of

B: 0.0005-0.0020 wt %,

Cu: 0.5 wt % or less,

Ni: 0.5 wt % or less,

Cr: 0.5 wt % or less, and

Mo: 0.5 wt % or less; and

the balance being Fe and incidental impurities (Claim 4).

5) Also, the present invention relates to a method for making a steelsheet for double-rolled tubes having excellent formability, andexcellent strength and toughness after forming and annealing a tubecomprising:

hot finish rolling of a steel material containing

C: 0.0005-0.020 wt %, and further containing one or two of

Nb: 0.003-0.040 wt %, and

Ti: 0.005-0.060 wt % at a final temperature of 1,000-850° C.; coiling at750° C. or less; cold rolling; continuous annealing at 650-850° C. for20 seconds or less; and second cold-rolling at a rolling reduction rateof 20% or less (Claim 5).

6) Further, the present invention relates to a method for making a steelsheet for double-rolled tubes, having excellent formability, andexcellent strength and toughness after forming and annealing a tube,comprising:

hot finish-rolling of a steel material at a final temperature of1,000-850° C., the steel material comprising

C: 0.0005-0.020 wt %,

S: 0.02 wt % or less, and

N: 0.0050 wt % or less, and further comprising one or two of

Nb: 0.003-0.040 wt %, and

Ti: 0.005-0.060 wt %,

each of the excessive Nb and Ti contents, calculated based on theassumption that TiN, TiS, TiC and NbC are formed as much as possible inthat order, being less than 0.005 wt %; coiling at 750° C. or less;cold-rolling; continuous annealing at 650° C.-850° C. for 20 seconds orless; and second cold-rolling at a rolling reduction rate of 20% or less(Claim 6).

7) Further, the present invention relates to a method for making a steelsheet for double-rolled tubes, described in the above 5) or 6), thesteel sheet comprising:

C: 0.0005-0.020 wt %,

Si: 0.10 wt % or less,

Mn: 0.1-1.5 wt %,

P: 0.02 wt % or less,

S: 0.02 wt % or less,

Al: 0.100 wt % or less, and

N: 0.0050 wt % or less; and further comprising one or two of

Nb: 0.003-0.040 wt %, and

Ti: 0.005-0.060 wt %, and

the balance being Fe and incidental impurities (Claim 7).

8) Also, the present invention relates to a method for making a steelsheet for double-rolled tubes, described in the above 5) or 6), thesteel sheet comprising:

C: 0.0005-0.020 wt %,

Si: 0.10 wt % or less,

Mn: 0.1-1.5 wt %,

P: 0.02 wt % or less,

S: 0.02 wt % or less,

Al: 0.100 wt % or less, and

N: 0.0050 wt % or less; and further comprising one or two of

Nb: 0.003-0.040 wt %, and

Ti: 0.005-0.060 wt %; and

at least one selected from the group consisting of

B: 0.0005-0.0020 wt %,

Cu: 0.5 wt % or less,

Ni: 0.5 wt % or less,

Cr: 0.5 wt % or less, and

Mo: 0.5 wt % or less; and

the balance being Fe and incidental impurities (Claim 8).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating the correlation between the Nb or Ticontent in a solid solution state and the ferrite grain size.

BEST MODE FOR CARRYING OUT THE INVENTION

The preferred embodiment of the present invention will now be described.

(1) Components in the Steel

C: 0.0005-0.020 wt %

An extremely reduced carbon content contributes to improved formability(decreased deformation stress and improved shape fixability) in the tubeproduction process. At a carbon content of less than 0.0005 wt %,however, coarsening of grains is prominent, hence desirable strength andtoughness are not achieved. Further, the possibility of the formation ofrough surfaces like the so-called "orange peel phenomenon" willincreases. On the other hand, a carbon content of more than 0.02 wt %causes a significant deterioration of ductility and shape fixability ofthe steel sheet, and thus deterioration of workability accompanied bythinning of the steel sheet is further prominent. Also, an excessivecarbon content leads to decreases in the cold-rolling performance. Thus,the carbon content is set to a range of 0.0005 to 0.020 wt %. It ispreferable that the range be 0.0010 to 0.015 wt % when requiring greaterstability of the mechanical properties and excellent ductility.

Si: 0.10 wt % or less

Addition of a large amount of Si causes decreased surface treatmentcharacteristics and corrosion resistance, significantly increases thestrength of the steel as a solid solution strengthening, and thus ailincreases the deformation resistance during the forming process. Thus,the upper limit is set to 0.10 wt %. It is preferable that the contentbe limited to 0.02 wt % or less when requiring particularly excellentcorrosion resistance.

Mn: 0.1-1.5 wt %

Manganese is an element effectively preventing hot cracking caused bysulphur. In particular, it is preferable that manganese be added tonon-titanium steel in response to the sulphur content. Since manganesecontributes to making grains finer and particularly to the suppressionof coarsening of grains when the steel is maintained at a hightemperature, the addition of manganese is preferred.

At least 0.1 wt % of manganese must be added in order to achieve theseadvantages. Since an excessive addition, however, leads to deteriorationof corrosion resistance and cold rolling characteristics because ofhardening of the steel sheet, the upper limit is set to 1.5 wt %. It ispreferable that manganese be added within a range of 0.60 wt % or lesswhen requiring more excellent corrosion resistance and formability.

P: 0.02 wt % or less

Phosphorus hardens the steel and causes deterioration of flangeworkability and shape fixability. Further, it is a harmful element whichcauses deterioration of corrosion resistance, hence the upper limit isset to 0.02 wt %. It is preferable that it be added in an amount of 0.01wt % or less when these characteristics are particularly important.

S: 0.02 wt % or less

Since sulphur is present as inclusions in the steel, and is an elementwhich causes the ductility of the steel to decrease and causes thedeterioration of corrosion resistance, the upper limit of the sulphurcontent is set to 0.02 wt %. It is preferable that it be added in amountof 0.01 wt % or less when particularly excellent workability isdemanded.

Al: 0.100 wt % or less

Aluminum is an element which is effective for deoxidation in steel.Since an excessive content, however, causes deterioration of surfacecharacteristics, the upper limit of the aluminum content is set to 0.100wt %. It is preferable that aluminum be added in an amount in the rangeof 0.008 to 0.060 wt % in view of stability of the mechanicalproperties.

N: 0.0050 wt % or less.

Nitrogen promotes occurrences of internal defects in the steel sheet aswell as slab cracking in a continuous casting process as the contentincreases. Since nitrogen causes excessive hardening of the steel, theupper limit is set to 0.0050 wt %. It is preferable that the nitrogencontent be 0.0030 wt % or less in view of stability of the mechanicalproperties and improvement in yield on account of the entire productionprocess.

Nb: 0.003-0.040 wt %

Niobium is an element which is effective for making the micro-structureof the steel sheet finer, and such an effect is held after heattreatment after tube production. Such a finer micro-structure in thesteel sheet causes significant improvement in secondary formability inthe use as a tube, such as bending and stretching of the tube, andimprovement in impact resistance. Such advantages due to niobium arenoticeable in a content of 0.003 wt % or more; however, the addition ofmore than 0.040 wt % will cause hardening of the steel and slabcracking, as well as deteriorated ductility during hot-rolling andcold-rolling. The niobium content is therefore set to a range of 0.003to 0.40 wt %. It is more preferable if the content be 0.020 wt % or lessin view of mechanical properties.

Ti: 0.005-0.060 wt %

Titanium is also effective for making the micro-structure finer as withniobium. Although it is added in an amount of 0.005 wt % or more toachieve such an effect, the addition of more than 0.060 wt % causes anincrease in the occurrence of surface defects. The titanium content istherefore set to a range of 0.005 to 0.060 wt %. It is more preferablethat the content be 0.015 wt % or less in view of mechanical properties.Niobium and titanium may be added solely or in combination, sinceeffects due to individual elements are not canceled by each other.

Nb and Ti in Solid Solution State

Niobium and titanium in solid solution state are a significantlyimportant feature in the present invention. Although the detailedmechanism has not been clarified, coarsening of the micro-structureafter forming-heat-treatment of the double-rolled tube can be remarkablysuppressed, as shown in FIG. 1, when at least one of niobium andtitanium in solid solution state is present in an amount of 0.005 wt %or more. The steel materials used in the experiment in FIG. 1 have thefollowing compositions; 0.0025 C-0.02 Si-0.5 Mn-0.01 P-0.010 S-0.040Al-0.0020 N-varied Nb or Ti, wherein the two levels of Nb contents, thatis, 0.018% and 0.015%, and two levels of Ti contents, that is, 0.040 and0.060% are employed. Conditions for hot-rolling and heat-treatment areas follows: the final temperature of hot-rolling is in the range of950-870° C., the coiling temperature is in the range of 720-540° C.,heat-treatment is performed at 750° C. for 20 sec, and secondcold-rolling of 2% is performed after the heat-treatment. As a result,the dissolved niobium content can be varied within the range of 0 to0.015%.

Among niobium and titanium, at least one element must be present,because the above-mentioned advantage is not achieved even if these twoelements are present in a total amount of 0.005 wt % or more. If each ofthese elements is present in a amount of 0.005 wt % or more, individualeffects due to these elements are not canceled by each other.Accordingly, it is important that at least one of niobium and titaniumis present in an amount of 0.005 wt % or more in a solid solution state.

The Nb or Ti content in a solid solution state is defined as thesubtraction of the precipitated Nb or Ti content, which is determined byelectrolytic analysis, from the total Nb or Ti content in the steel. Theelectrolytic analysis is defined as an analytical method by means ofconstant potential electrolysis in a non-aqueous electrolyte, wherein asample is electrolyzed in a 10% acetylacetone -1% tetramethylammoniumchloride electrolyte, the residue is collected on a 0.2-μm nuclear porefilter, and the relevant elements are determined by an absorptiometricmethod.

Excessive Ti and Nb

As described above, although titanium and niobium are essential elementsin the present invention, the addition of an excessive amount of eachelement causes the following disadvantages.

In general cold-rolled steel sheets, titanium and niobium are consideredas elements which are desirable for improving formability, such assoftening, and for improving the r value and ductility. In ultra-thinsteel sheets in the present invention, however, an extremely highcold-rolling reduction rate is required in the production step (at least70% and generally 80% or more in the current highest technology for thinhot-rolling), hence a large load occurs during cold rolling. Theaddition of an excessive amount of Nb or Ti therefore causes thedeformation resistance to increase significantly during rolling andcauses deterioration of surface characteristics. Changes in mechanicalproperties, such as strength, an r value, and ductility, between theworking directions, that is, anisotropy are also increased. The additionof an excessive amount of Ti or Nb must be avoided to prevent theoccurrence of the above-mentioned disadvantages. Further, it ispreferable that the Ti and Nb contents be minimized within necessity inview of the material costs.

Based on the grounds described above, the present inventors studied theupper limits of the Ti and Nb contents by the precipitation process, anddiscovered the following upper limits of the contents. Each of theexcessive Nb and Ti contents, which are calculated using the contents inthe steel based on the assumption that TiN, TiS, TiC and NbC are formedas much as possible in that order, must be less than 0.005 wt %.

Specifically, the excessive Ti content (hereinafter referred to asTi_(ex)) means the residual Ti content by weight percent after theformation of TiN, TiS and TiC, and is stoichiometrically calculated bythe following equation:

    Ti.sub.ex =Ti-(48/14).N-(48/32).S-(48/12).C

The excessive Nb content (hereinafter referred to as Nb_(ex)) iscalculated as follows:

1) When titanium is not added, Nb_(ex) is calculated by the followingequation in consideration of only NbC, because TiN, TiS or TiC is notformed:

    Nb.sub.ex =Nb-(93/12).C

2) When titanium is added and when Ti_(ex) ≧0, Nb_(ex) is calculated bythe following equation, because residual carbon forming NbC is notpresent:

    Nb.sub.ex =Nb

3) When titanium is added and when Ti_(ex) ≦0, first, the Ti content asthe formed TiN and TiS (hereinafter referred to as TiNS) is calculatedby the following equation:

    Ti.sub.NS =Ti-(48/14).N-(48/32).S,

and then Nb_(ex) is calculated by either of the following equations inresponse to the Ti_(NS) content:

3a) when Ti_(NS) ≦0,

Nb_(ex) =Nb-(93/12).C (the same as the above-mentioned 1)), because allthe carbon is used for the formation of NbC, or

3b) when Ti_(NS) >0,

Nb_(ex) =Nb-(93/12).(C-(12/48).Ti_(NS)), because after TiC is formed inresponse to the Ti_(NS), the residual carbon is used for the formationof NbC.

By providing such upper limits of the Ti and Nb contents, it isdifficult to maintain the contents of solid solutions. The presentinvention is, however, characterized in that desirable contents ofdissolved Ti and Nb are achieved, the problems in the steel sheetproduction are solved, and compatibility between the mechanicalproperties and a given strength and a given toughness after forming adouble-rolled tube is achieved.

The steel sheet may contain at least one component selected from a groupor groups consisting of B: 0.0005-0.0020 wt % (group A), Cu: 0.5 wt % orless, Ni: 0.5 wt % or less, Cr: 0.5 wt % or less, and Mo: 0.5 wt % orless (group B, hereinafter the same).

B: 0.0005-0.0020 wt %

B is an element which is effective for maintaining strength because of afiner structure after making the tube. Such an advantage is recognizedby the addition of 0.0005 wt % or more, whereas the addition of morethan 0.0020 wt % causes undesirable increase in planar anisotropy of thesteel sheet. Accordingly, the B content is added within the range of0.0005 to 0.0020 wt %, and preferably 0.0005 to 0.0010 wt %.

Cu: 0.5 wt % or less, Ni: 0.5 wt % or less,

Cr: 0.5 wt % or less and Mo: 0.5 wt % or less

These elements, which enhance the strength of the steel sheet, andparticularly, the strength after heat treatment in the brazing of thetube, are added, if necessary. When each of these elements is, however,added in an amount of more than 0.5 wt %, cold-rolling characteristicsdeteriorate, hence they are added within 0.5 wt % or less.

The group A element including B, and the group B elements including Cu,Ni, Cr and Mo, both being optional components, may be added solely or incombination, which consists of at least two elements from the same groupor different groups.

(2) Regarding Crystal Structure etc.:

The grain size of the ferrite is set to 5 to 10 μm. The steel containingthe crystals having a size of less than 5 μm is hardened, henceunsatisfactory phenomena, such as a poor shape after tubing and severeabrasion of tools, significantly occur. On the other hand, when thegrain size is more than 10 μm, a uniformly fine texture is barelymaintained after forming-annealing, hence the strength and toughness ofthe product in use decrease. Accordingly, the crystal grain size in thesteel sheet is controlled to within 5 to 10 μm.

It is desirable that the hardness (temper grade) be T1-T3. A tempergrade of more than T3 evidently causes deterioration of formability andcauses significant decrease in the life of the tools. It is desirablethat the strength of the raw material be as low as possible if thestrength after forming and heat treatment of the tube is sufficientlyhigh.

Toughness, as well as the strength, after forming and heat treatment ofthe tube of the steel sheet for double-rolled tubes is also an importantfactor. The toughness is evaluated by tensile testing or high-speedtensile testing of a pipe with a notch.

(3) Manufacturing Conditions etc.:

Hot Finish-Rolling:

Since uniformity of the micro structure after annealing decreases whenthe final rolling temperature in the hot finish-rolling is lower than850° C. and such nonuniformity is succeeded after annealing after coldrolling, a remarkable fluctuation of the mechanical properties isrecognized, resulting in decreased reliability of mechanical properties.On the other hand, surface flaws due to scales prominently occur at atemperature of higher than 1,000° C. Accordingly, it is desirable thatthe final rolling temperature of the hot finish-rolling be within therange of 1,000-850° C. It is preferable that the final temperature bewithin the range of 950-850° C. in view of hot-rolling characteristics.

To decrease the opportunity of precipitation of Ti or Nb after hotfinish-rolling, it is preferable that the steel sheet be quenched at aquenching rate of 30° C./sec. or more within a second after the finishrolling.

In finish-rolling the sheet bar after hot rough-rolling, adaption ofcontinuous rolling (endless rolling) including joining the sheet bars atthe inlet side of a finish-rolling mill is preferable, becausetravelling of the front and rear ends of the steel sheet is stabilizedand thus rapid cooling of the steel sheet immediately afterfinish-rolling can be achieved over the entire length.

Coiling after Hot-Rolling:

It is difficult to maintain Nb and Ti in solid solution state in thesteel if the coiling temperature after hot rolling is higher than 750°C. As a result, suppression of coarsening of crystal grains due todissolved Nb and Ti cannot be sufficiently achieved. In this case, it isdifficult to achieve uniform mechanical properties in the longitudinaldirection. Accordingly, the coiling temperature after hot-rolling is setto 750° C. or less, and preferably, 650° C. or less.

Conditions of the following pickling and cold-rolling are not fixed andare determined according to a general method for making an ultra-thinsteel sheet.

Annealing after Cold-Rolling:

If the annealing temperature is lower than 650° C., most of thestructure is. occupied by a non-recrystallized structure, and thus thesteel sheet is not softened. A target, that the load is reduced in thetube production process, is therefore not achieved. Although annealingat 650° C. or more does not form a perfect recrystallized structure,softening which is sufficient to the usage in the present invention isachieved. At an annealing temperature at 750° C. or more, most of thestructure is occupied by a recrystallized structure, and extremelysuperior workability is achieved. When it is annealed at a temperaturehigher than 850° C. as in general ultra-low carbon cold-rolled steelsheets for working, the micro structure in the steel is coarsened andbecomes non-uniform, the precipitation of Ti and Nb is promoted duringthe annealing, and thus a uniform and fine texture is not formed aftertubing-heat-treatment.

Accordingly, the annealing temperature is within the range of,preferably, 650-850° C., and particularly, 700-800° C. in view ofstability of the mechanical properties. It is more preferable that thetemperature be 750° C. or less in economical view, in addition tostability of the mechanical properties.

The soaking time during annealing is also an important factor.Conventional annealing is generally performed for at least 30 seconds inorder to form a stable recrystallization texture. Such annealing,however, does not form dissolved Ti or Nb, which is essential for thepresent invention, because of the precipitation of Ti and Nb during theannealing. The dissolved Ti or Nb can be formed by controlling theannealing temperature to 850° C. or less and the soaking time to 20seconds or less, as described above. It has been considered that theannealing of an ultra-low carbon steel sheet for such a short time hasan unsatisfactory r value and ductility for the use with deep drawing;however, such annealing for a short time can be applied to the presentinvention without any problems.

Second Cold-Rolling after Annealing

Second cold-rolling performed after annealing controls surface roughnessand decreases the thickness of the sheet. It is preferable that thereduction rate of the second cold-rolling be 1.0% or more. If the secondcold rolling is performed at a reduction rate of more than 20%, tubeforming characteristics deteriorate because of increased yield stressamong mechanical properties. Accordingly, the reduction rate of thesecond cold rolling after annealing is set to 20% or less. It ispreferable that the reduction rate be within the range of 1.0 to 10%.

A steel sheet in accordance with the present invention is manufacturedby following the above-mentioned steps. The final thickness of the steelsheet is not limited, and the present invention is more effectivelyapplied to a final thickness of 0.35 mm or less.

Surface Treatment:

A metal having a self-brazing property, such as copper, is plated ontothe above-mentioned steel sheet which is brazed by heat treatment afterthe tube production. Although no additional surface treatment isbasically required, chemical or electrochemical treatment may be addedto enhance the metal plating effect.

EXAMPLE 1

A series of steels containing the components shown in Table 1 and thebalance being Fe were melted in a converter, and each of the resultingsteel slabs was hot rolled under the condition shown in Table 2 (rapidcooling at a rate of 50° C./sec. within 0.5 seconds after finishing thehot-rolling. In the hot-rolling, a slab having a thickness of 260 mm wasrough-rolled with seven passes to form a sheet bar having a thickness of30 mm, and a hot-rolled mother sheet coil was made from the sheet barwith a 7-stand tandem rolling mill. The mother sheet coil was pickled,cold-rolled with a tandem rolling mill, annealed and subjected to secondcold-rolling.

Copper with a thickness of 30 μm was electroplated onto the steel sheet,and a 3.45 mmφ double-rolled tube was formed with the plated sheet by aconventional process, subjected to 5% drawing, and heat-treated at1,120° C. for 20 seconds to braze the copper plating layer.

The resulting steel sheet and the self-brazed double-rolled tube wereused for the following tests:

1) The grain size of the ferrite crystals at the transversecross-section;

2) Tensile strength by a static tensile test;

3) The reduction of area by a low-temperature tensile test (at -40° C.)for evaluating toughness, which is equivalent to impact tensile strengthat a high speed; and

4) Bend test (180° bending).

In all these tests, general methods for determining mechanicalproperties were used except that the double-rolled tube was used withoutfurther working.

The results are shown in Table 3. In each of the examples in accordancewith the present invention in which the dissolved Nb and Ti contents liewithin adequate ranges, the crystal grains are not coarsened after hightemperature heating, sufficient strength and ductility, excellentlow-temperature toughness (drawing due to the tensile test), excellentbend workability, and excellent shape fixability are achieved.

Each of the steels 12, 13 and 14 is hard, a satisfactory shape is notachieved in the final cold-rolled steel sheet, with inferior bendingcharacteristics.

EXAMPLE 2

A series of slabs having a composition shown in No. 1 of Table 1 washot-rolled, pickled, cold-rolled, and subjected to continuous annealingand second cold-rolling under the conditions shown in Table 4 (thecooling condition was the same as in Example 1) to form ultra-thincold-rolled steel sheets. A conventional box-annealed low-carbonaluminum-killed steel was used for comparison.

Copper was plated onto the surface of each of these steel sheets as inExample 1 to form a double-rolled tube.

Abrasion of the tools (life of the tools) used in the tubing was alsoevaluated in addition to the tests in Example 1. In the evaluation ofthe life of the tools, the relative ratio, in which the life of thesample for comparison (box-annealed low-carbon aluminum-killed steel)was set to 1, was used.

The experimental results are also shown in Table 4. Table 4 demonstratesthat each of the soft steel sheets in accordance with the presentinvention has a life of the tools which is approximately 1.5 times thatin the sample for comparison. In the samples containing dissolved Nb andTi within the range of the present invention, coarsening of themicrostructure is effectively suppressed after tubing.

INDUSTRIAL APPLICABILITY

As described above, since the steel sheet in accordance with the presentinvention is soft, it has low deformation resistance, and reducesabrasion of the tools and thus prolongs their life. In the presentinvention, in addition to excellent formability, a double-rolled tubehaving excellent strength and toughness is produced due to reducedcoarsening of the ferrite grains.

Further, a continuous annealing process is applied in the presentinvention, hence a high production efficiency and uniformity ofmechanical properties can be achieved.

Accordingly, a high-quality double-rolled tube with high airtightnesscan be effectively and economically produced in the present invention.

                                      TABLE 1                                     __________________________________________________________________________    (wt %)                                                                                                              Excessive                                                                          Excessive                          No.                                                                              C   Si Mn P   S  Al N   Nb Ti Others                                                                             Nb   Ti   Remarks                       __________________________________________________________________________     1 0.0020                                                                            0.01                                                                             0.55                                                                             0.010                                                                             0.005                                                                            0.055                                                                            0.0015                                                                            0.017                                                                            --        0.0015                                                                           --   Within this                                                                   invention                      2 0.0040                                                                            0.01                                                                             0.15                                                                             0.005                                                                             0.005                                                                            0.040                                                                            0.0015                                                                            0.009                                                                            --      -0.022                                                                             --   Within this                                                                   invention                      3 0.0120                                                                            0.01                                                                             0.20                                                                             0.006                                                                             0.003                                                                            0.065                                                                            0.0018                                                                            0.015                                                                            -- Cu: 0.05                                                                           -0.078                                                                             --   Within this                                                                   invention                      4 0.0080                                                                            0.01                                                                             0.10                                                                             0.01                                                                              0.007                                                                            0.045                                                                            0.0025                                                                            0.018                                                                            -- Cr: 0.06                                                                           -0.044                                                                             --   Within this                                                                   invention                      5 0.0020                                                                            0.01                                                                             0.15                                                                             0.007                                                                             0.008                                                                            0.065                                                                            0.0021                                                                            -- 0.015                                                                            Ni: 0.06,                                                                          --   -0.12                                                                              Within this                                                    Mo: 0.02       invention                      6 0.0035                                                                            0.01                                                                             0.35                                                                             0.006                                                                             0.005                                                                            0.040                                                                            0.0040                                                                            -- --      --   --   For comparison                 7 0.0600                                                                            0.01                                                                             0.15                                                                             0.006                                                                             0.002                                                                            0.040                                                                            0.0010                                                                            -- --      --   --   For comparison                 8 0.0410                                                                            0.01                                                                             2.00                                                                             0.006                                                                             0.002                                                                            0.040                                                                            0.0010                                                                            -- --      --   --   For comparison                 9 0.0020                                                                            0.01                                                                             0.57                                                                             0.011                                                                             0.006                                                                            0.053                                                                            0.0012                                                                            0.017                                                                            -- B: 0.0009                                                                            0.0015                                                                           --   Within this                                                                   invention                     10 0.0020                                                                            0.02                                                                             0.17                                                                             0.0010                                                                            0.008                                                                            0.067                                                                            0.0021                                                                            -- 0.015   --   -0.12                                                                              Within this                                                                   invention                     11 0.0020                                                                            0.03                                                                             0.22                                                                             0.0009                                                                            0.006                                                                            0.052                                                                            0.0021                                                                            0.017                                                                            0.015     0.0015                                                                           -0.009                                                                             Within this                                                                   invention                     12 0.0020                                                                            0.01                                                                             0.20                                                                             0.0070                                                                            0.007                                                                            0.040                                                                            0.0020                                                                            0.025                                                                            0.035     0.025                                                                              0.009                                                                            Within this                                                                   invention                     13 0.0020                                                                            0.02                                                                             0.40                                                                             0.0050                                                                            0.005                                                                            0.040                                                                            0.0022                                                                            0.026                                                                            --        0.011                                                                            --   For comparison                14 0.0022                                                                            0.01                                                                             0.22                                                                             0.0030                                                                            0.006                                                                            0.050                                                                            0.0021                                                                            -- 0.039   --     0.014                                                                            For comparison                __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________    Hot finish rolling  Cold rolling                                                                            Continuous annealing                                                                          2nd cold                           Final Final                                                                              Coiling                                                                             Rolling                                                                            Final            Soaking                                                                           rolling                            Temperature                                                                         thickness                                                                          Temperature                                                                         reduction                                                                          thickness  Temperature                                                                         time                                                                              reduction                       Steel                                                                            (° C.)                                                                       (mm) (° C.)                                                                       (%)  (mm) Method                                                                              (° C.)                                                                       (sec)                                                                             (%)                             __________________________________________________________________________    1˜6                                                                        890   2.1  650   85   0.315                                                                              Continuous                                                                          760   15  2                               9˜14                                                                    7  860   2.1  650   85   0.315                                                                              Continuous                                                                          680   30  2                               8  860   2.1  600   85   0.315                                                                              Box   650   55 (hr)                                                                           2                               __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________                     Crystal grain   Reduction                                                     size    Tensile strength                                                                      of area at                                                    (μm) (kgf/mm.sup.2)                                                                        low                                                           Before                                                                            After                                                                             Before                                                                            After                                                                             temperature                                                   heat                                                                              heat                                                                              heat                                                                              heat                                                                              tensile                                          Dissolved Nb                                                                         Dissolved Ti                                                                        treat-                                                                            treat-                                                                            treat-                                                                            treat-                                                                            testing                                      Steel                                                                             (wt %) (wt %)                                                                              ment                                                                              ment                                                                              ment                                                                              ment                                                                              (%)    Cracks after bend                                                                             Remarks               __________________________________________________________________________     1  0.009        8.1 25  37  35  85     Non             Within this                                                                   invention              2  0.006        8.5 26  38  36  80     Non             Within this                                                                   invention              3  0.008        7.9 24  39  39  82     Non             Within this                                                                   invention              4  0.01         7.9 23  38  37  82     Non             Within this                                                                   invention              5         0.007 8.0 21  38  37  85     Non             Within this                                                                   invention              6               8.3 90  36  25  85     Cracks & rough surface                                                                        For comparison         7               8.2 25  41  31  60     Non             For comparison         8               8.0 27  36  30  58     Non             For comparison         9  0.009        7.9 22  38  37  84     Non             Within this                                                                   invention             10         0.006 8.4 26  35  34  88     Non             Within this                                                                   invention             11  0.008  0.006 7.8 22  36  35  89     Non             Within this                                                                   invention             12  0.022  0.007 7.6 24  37  36  70     Fine cracks, surface flaws, and                                               large           For comparison                                                rolling load (deteriorated shape)     13  0.021        8.1 25  35  34  72     Fine cracks, surface flaws, and                                               large           For comparison                                                rolling load (deteriorated shape)     14         0.009 8.2 28  34  31  70     Fine cracks, surface flaws, and                                               large           For comparison                                                rolling load (deteriorated            __________________________________________________________________________                                            shape)                            

                                      TABLE 4                                     __________________________________________________________________________                                                  Crystal grain                   Hot finish                                    size                            rolling      Cold rolling        2nd cold     (μm)                                Final                                                                            Coil-                                                                            Rolling                                                                           Final                                                                            Continuous                                                                              Soak-                                                                            rolling                                                                            Dis-                                                                              Dis-                                                                              Before                                                                            Before                      Final  thick-                                                                           ing                                                                              reduc-                                                                            thick-                                                                           annealing ing                                                                              reduc-                                                                             solved                                                                            solved                                                                            heat                                                                              heat                                                                              Life                       Temp.                                                                             ness                                                                             Temp                                                                             tion                                                                              ness     Temp.                                                                             time                                                                             tion Nb  Ti  treat-                                                                            treat-                                                                            of                      Steel                                                                            (° C.)                                                                     (mm)                                                                             (° C.)                                                                    (%) (mm)                                                                             Method                                                                              (° C.)                                                                     (sec)                                                                            (%)  (wt %)                                                                            (wt %)                                                                            ment                                                                              ment                                                                              tools                                                                            Remarks              __________________________________________________________________________    1  890 2.2                                                                              650                                                                              85  0.33                                                                             Continuous                                                                          760 15 2.0  0.010                                                                             0   8.5 27  1.5                                                                              Within this                                                                   inveniton            5  890 2.1                                                                              650                                                                              84  0.34                                                                             Continuous                                                                          765 15 2.0  0.000                                                                             0.012                                                                             9.2 26  1.5                                                                              Within this                                                                   inveniton            1  880 2.2                                                                              780                                                                              85  0.33                                                                             Continuous                                                                          760 18 1.5  0.001                                                                             0   9.2 45  1.6                                                                              For                                                                           comparison           1  800 2.1                                                                              650                                                                              84  0.34                                                                             Continuous                                                                          860 18 1.5  0.001                                                                             0   10.5                                                                              50  1.6                                                                              For                                                                           comparison           1  880 2.7                                                                              600                                                                              84  0.43                                                                             Continuous                                                                          760 15 25.0 0.009                                                                             0   8.7 45  1.2                                                                              For                                                                           comparison           6  860 2.1                                                                              620                                                                              84  0.34                                                                             Continuous                                                                          700 18 1.5  0.000                                                                             0   10.6                                                                              100 1.6                                                                              For                                                                           comparison           7  840 2.2                                                                              650                                                                              85  0.33                                                                             Continuous                                                                          700 18 1.5  0.000                                                                             0   7.2 21  0.75                                                                             For                                                                           comparison           8  840 2.2                                                                              600                                                                              85  0.33                                                                             Box   680 -- 1.8  0.000                                                                             0   8.5 23  1.0                                                                              For                                                                           comparison           1  840 2.1                                                                              700                                                                              85  0.32                                                                             Continuous                                                                          720 45 2.0  0.000                                                                             0   8.7 95  1.6                                                                              For                                                                           comparison           __________________________________________________________________________

What is claimed is:
 1. A steel sheet for double-rolled tubes havingexcellent formability, and excellent strength and toughness afterforming and heat treatment of a tube comprising:C: 0.0005-0.020 wt %;and further comprising one or two of Nb: 0.003-0.040 wt %, and Ti:0.005-0.060 wt %;at least one of Nb and Ti being present in a solidsolution state in an amount of 0.005 wt % or more, the crystal grainsize in the ferrite structure being in the range of 5 to 10 μm.
 2. Asteel sheet for double-rolled tubes having excellent formability, andexcellent strength and toughness after forming and heat treatment of atube comprising:C: 0.0005-0.020 wt %, S: 0.02 wt % or less, and N:0.0050 wt % or less; and further comprising one or two ofNb: 0.003-0.040wt %, and Ti: 0.005-0.060 wt %;each of the excessive Nb and Ti contents,calculated based on the assumption that TiN, TiS, TiC and NbC are formedas much as possible in that order, being less than 0.005 wt %, at leastone of Nb and Ti being present in solid solution state in an amount of0.005 wt % or more, the crystal grain size in the ferrite structurebeing in the range of 5 to 10 μm.
 3. A steel sheet for double-rolledtubes, according to claim 1, comprising:C: 0.0005-0.020 wt %, Si: 0.10wt % or less, Mn: 0.1-1.5 wt %, P: 0.02 wt % or less, S: 0.02 wt % orless, Al: 0.100 wt % or less, and N: 0.0050 wt % or less; and furthercomprising one or two ofNb: 0.003-0.040 wt %, and Ti: 0.005-0.060 wt %;andthe balance being Fe and incidental impurities.
 4. A steel sheet fordouble-rolled tubes, according to claim 1, comprising:C: 0.0005-0.020 wt%, Si: 0.10 wt % or less, Mn: 0.1-1.5 wt %, P: 0.02 wt % or less, S:0.02 wt % or less, Al: 0.100 wt % or less, and N: 0.0050 wt % or less;and further comprising one or two ofNb: 0.003-0.040 wt %, and Ti:0.005-0.060 wt %; andat least one selected from the group consisting ofB: 0.0005-0.0020 wt %, Cu: 0.5 wt % or less, Ni: 0.5 wt % or less, Cr:0.5 wt % or less, and Mo: 0.5 wt % or less; andthe balance being Fe andincidental impurities.
 5. A method for making a steel sheet fordouble-rolled tubes, having excellent formability, and excellentstrength and toughness after forming and heat treatment of a tube,comprising:hot finish-rolling of a steel material at a final temperatureof 1,000-850° C., the steel material comprisingC: 0.0005-0.020 wt %, S:0.02 wt % or less, and N: 0.0050 wt % or less, and further comprisingone or two ofNb: 0.003-0.040 wt %, and Ti: 0.005-0.060 wt %,each of theexcessive Nb and Ti contents, calculated based on the assumption thatTiN, TiS, TiC and NbC are formed as much as possible in that order,being less than 0.005 wt %; coiling at 750° C. or less; cold-rolling;continuous annealing at 650° C.-850° C. for 20 seconds or less; andsecond cold-rolling at a rolling reduction rate of 20% or less.
 6. Amethod for making a steel sheet for double-rolled tubes, according toclaim 5, the steel sheet comprising:C: 0.0005-0.020 wt %, Si: 0.10 wt %or less, Mn: 0.1-1.5 wt %, P: 0.02 wt % or less, S: 0.02 wt % or less,Al: 0.100 wt % or less, and N: 0.0050 wt % or less; and furthercomprising one or two ofNb: 0.003-0.040 wt %, and Ti: 0.005-0.060 wt %;andthe balance being Fe and incidental impurities.
 7. A method formaking a steel sheet for double-rolled tubes, according to claim 5, thesteel sheet comprising:C: 0.0005-0.020 wt %, Si: 0.10 wt % or less, Mn:0.1-1.5 wt %, P: 0.02 wt % or less, S: 0.02 wt % or less, Al: 0.100 wt %or less, and N: 0.0050 wt % or less; and further comprising one or twoofNb: 0.003-0.040 wt %, and Ti: 0.005-0.060 wt %; andat least oneselected from the group consisting of B: 0.0005-0.0020 wt %, Cu: 0.5 wt% or less, Ni: 0.5 wt % or less, Cr: 0.5 wt % or less, and Mo: 0.5 wt %or less; andthe balance being Fe and incidental impurities.
 8. A steelsheet for double-rolled tubes, according to claim 2, comprising:C:0.0005-0.020 wt %, Si: 0.10 wt % or less, Mn: 0.1-1.5 wt %, P: 0.02 wt %or less, S: 0.02 wt % or less, Al: 0.100 wt % or less, and N: 0.0050 wt% or less; and further comprising one or two ofNb: 0.003-0.040 wt %, andTi: 0.005-0.060 wt %; andthe balance being Fe and incidental impurities.9. A steel sheet for double-rolled tubes, according to claim 2,comprising:C: 0.0005-0.020 wt %, Si: 0.10 wt % or less, Mn: 0.1-1.5 wt%, P: 0.02 wt % or less, S: 0.02 wt % or less, Al: 0.100 wt % or less,and N: 0.0050 wt % or less; and further comprising one or two ofNb:0.003-0.040 wt %, and Ti: 0.005-0.060 wt %; andat least one selectedfrom the group consisting of B: 0.0005-0.0020 wt %, Cu: 0.5 wt % orless, Ni: 0.5 wt % or less, Cr: 0.5 wt % or less, and Mo: 0.5 wt % orless; andthe balance being Fe and incidental impurities.